The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday October 12 2005

In the article below, we mistakenly divide the three branches of life on Earth into bacteria, archaea and prokaryotes. In fact, bacteria and archaea are both groups of prokaryotes, having no nucleus and all their genetic material in a single filament of DNA. Everything else is a eukaryote, having cells with a nucleus containing genetic material.

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Call it the ultimate search in genealogy - the hunt for a lifeform that existed more than 3bn years ago and is the common ancestor of everything alive today. The scientists on this search might not agree what the organism might be, but they already have a name: Luca (Last Universal Common Ancestor). Evolutionary biologists who believe that Luca did exist argue that everything from bacteria to birch trees to blue whales is descended from this one living thing.

It's a controversial topic. Not least because some scientists suggest that, if Luca existed, the evolutionary processes at work on primordial Earth might have been quite different to the Darwinian model of natural selection. That's scientific heresy, but it is not stopping the search. At the National Centre for Biotechnology Information in Maryland, for example, a team led by the evolutionary biologist Eugene Koonin is trying to identify Luca's genes. So far they have 600 possibles. Once they can pare it down to the bare minimum, they hope to assemble Luca's genome in the lab.

Despite its differences from Darwinism, Luca is descended from Darwin's theory of evolution by natural selection. Darwin proposed that similar species must have had common ancestors. Each species belonged to a huge family tree but even Darwin was unable to work out whether those trees were connected.

After Watson and Crick's discovery of the structure of DNA and the subsequent birth of molecular biology, scientists began to unpick the differences between different species. But as well as differences, they found similarities - the common molecules of life such as DNA, amino acids and proteins - that suggested differing forms of life were more closely linked than even Darwin had thought. But the latest research on Luca suggests another scientific heresy, this time concerning DNA. It has long been thought that Luca existed when genes were made from DNA's less sophisticated cousin, RNA. But some scientists now believe Luca was DNA-based, suggesting DNA evolved twice.

So what was Luca? The answer depends on which of the three branches of life on Earth is shown to be the oldest - bacteria, archaea (single-celled organisms such as the extremophiles, which can endure incredibly high temperatures and pressures) and prokaryotes (everything else, including all plants, animals and fungi). The answers could be decades or even centuries away but there's no real rush among scientists. Luca has been waiting to be discovered for some time already.